The present invention relates generally to satellite navigation, and more particularly, to a method and apparatus for altering the coordinates obtained by satellite navigation in a receiving system to fix an untrue position.
Several satellite navigation systems are currently in operation and are being employed in numerous applications. Exemplary systems include the xe2x80x9cGPS-Navstarxe2x80x9d and xe2x80x9cGlonass,xe2x80x9d both which are particularly modern and precise. Moreover, new satellite navigation systems or extensions thereof are currently in the planning stage, e.g., the European system xe2x80x9cENSS.xe2x80x9d With the assistance of these systems, position, speed and course can be determined with high precision and low cost with receiving and evaluating equipment (navigation receivers). The development of these high precision satellite navigation systems pose an immediate threat as navigation receivers are increasingly employed in weapons systems or used for other military purposes by a potential enemy.
Countermeasures for reducing the effectiveness of these threatening forces or weapons systems are known. Typically, they include eliminating the receiving power or manipulating the position-finding capability of the enemy""s navigation receiver. The effectiveness cannot generally be restored, and if so, only through the use of other navigational methods in combination with the navigation receiver. Such substitute satellite navigation methods do not generally achieve comparable precision with similarly low cost.
Exemplary methods exist for eliminating reception capability by irradiating energy in the frequency bands of the satellite transmitters with jamming transmitters. Achieving the necessary reception field strengths for a limited area around the jamming transmitter dose not present a problem if the energy supply of the transmitter can be guaranteed. For example, unmodulated or noise-modulated jamming methods are possible. However, this approach generally exhibits lower efficiency than those that take into account the modulation method and the structure of the modulated satellite codes. Unfortunately, the use of satellite codes is only possible if information on these systems is available to one seeking to prevent satellite navigation. Therefore, as a precaution, satellite providers often use secret codes. This occurs, firstly, in order to prevent undesired use of the navigation system and, secondly, to make the possibility of interference with manipulation more difficult.
One precaution adopted by the manufacturers and users of the navigation systems is to check the integrity of the received signals. Hence, if the received signals cannot be evaluated, are contradictory or untrustworthy, use of the satellite navigation system will be suspended. In the case of simple jamming, without the use of the codes, this would generally be the case. Furthermore, received signals that cannot be evaluated often occur in terrain that is prone to multipath propagation or in sites that are partially shaded from radiation, or even conceivably in areas affected by local interference, without there being any deliberate attempt to jam. For these reasons, conventional automatic navigation systems are designed to bridge such situations when signals cannot be evaluated for a certain time, thereby rendering simple jamming techniques unacceptable in many countermeasure applications.
Conventional navigation receivers are also capable of suppressing simple interference even when the strength of the reception field of the interference signal is greater than that of the desired signal. For this purpose a so-called adaptive null steering of the antenna system can be applied, which assigns very low gain to one direction of the radiation pattern.
Increased reliability is achieved through common evaluation of different signals and frequency bands or by employing more than one satellite system for obtaining the navigational information. Only a slight advantage is achieved over deliberate jamming.
As a result of these conventional techniques employed in navigational systems, countermeasures for the effective hostile use of such systems face a number of problems.
First, when confronted by a simple jamming measure, a receiving system is not provided with a plausible received signal, and therefore, measures to protect the integrity may take effect.
Second, for reasons of secrecy or modifiability, the copying of satellite signal structure without the cooperation of the provider is only possible in the case of published civil codes. Generating replicas jointly with the satellite provider requires substantial logistic support and input in terms of maintaining secrecy as well as constant updating in accordance with the provider""s agreement. This cannot always be guaranteed.
Third, the use of simple jamming methods can generally be counteracted with conventional means of radio reconnaissance if these reach the propagation range of the jamming signal.
Finally, direction-selecting precautions on the part of the hostile receiver, in particular, would be able to increase the jamming output required for simple interference to such an extent that an undesirably large area is affected by the interference and that the interference in turn becomes easier to counteract. Accordingly, there is a current need for a method and apparatus which, instead of preventing the satellite signal from being received or evaluated by simple jamming, strives to alter the position fixed by satellite navigation in enemy navigation receivers from the true coordinates obtained without detectable intervention.
The present invention is directed to a method and apparatus which satisfies this need. There is, therefore provided, in accordance with a preferred embodiment of the present invention, a system and method for deceiving a satellite navigation receiver. A receiving site includes a receiver which receives satellite signals sufficient to fix the position of the receiving site. An emission site with a transmitter receives the satellite signals from the receiver site and transmits them into a zone such that a navigation receiver, if it were located in the zone, would detect the position of the receiver site from the transmitted satellite signals.
This arrangement has a number of applications. By way of example, a target can be masked by locating the target within the zone so that a navigation receiver would not detect the presence of the target when the navigation receiver is in the zone. Alternatively, a pseudo target can be generated by locating the target at the receiving site so that the navigation receiver detects the presence of the target when the navigation receiver is within the zone created by the transmitter at the emission site away from the receiver site.
In an alternative embodiment, a number of receiver sites are employed. Each receiver site has a receiver for receiving satellite signals sufficient to fix the position of its respective receiving site. The received satellite signals from each receiver is transferred to a corresponding emission site where the signals are transmitted into a zone. As the navigation receiver advances through the zones established by the emission sites speed or course information is altered.
The described embodiments offer an attractive solution to simple jamming techniques in that a hostile navigation receiver is provided with a plausible signal, which despite integrity testing is recognized as trustworthy but nevertheless generates incorrect navigational information. xe2x80x9cManipulation,xe2x80x9d defined herein as the simulation of untrue coordinates, enables a number of the enemy""s precautions and integrity protection measures to be overcome.
Moreover, only a few, generally public or easy to acquire, parameters of the transmitting signal (frequency, bandwidth, approximate field strength)are required. No codes are needed. Manipulation is therefore possible without the provider""s cooperation. The fact that the cooperation of the satellite provider is not required obviates the need for the aforementioned logistic input and dispenses with the problems of updating.
Another advantage of a preferred embodiment is that it generally requires less transmission power than simple jamming measures. For this reason and because of their signal structure they are more difficult to counteract.
In addition, if the null steering on the receiver""s side is oriented away from a plausible incoming signal, then this protective measure can be overcome.
It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only embodiments of the invention by way of illustration of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.